In recent years, polylactic acid, polyglycolic acid, a copolymer of polylactic acid and polyglycolic acid and like biodegradable and bioabsorbable polymers are applied to bone joining devices as substitute material for metals, ceramics etc. since there is no need to remove the bone joining material after operation.
These bone joining devices are produced using fusion-molded articles which are formed, usually by cutting, into various shapes such as screws, nails, pins, tapping screws, plates, bolts, rivets, staples, washers, anchors, wires, etc.
However, the above method requires an advanced cutting technique and considerable time. When the molded articles are stretched products having improved flexural strength and tensile strength, formed fibrils or oriented molecules are cut by the cutting operation, thereby causing the problems of lowered strength and accelerated degradation rate of the articles. Furthermore, the head portion of the above bone joining devices usually has a larger diameter than its shank portion. Therefore, it has been problematic that a great amount of cut dust is produced by cutting a molded article having the diameter of the head portion to form the narrower shank portion.
Further, the bone joining devices, in case of a screw, have a drawback of impaired screw thread strength since fibrils or oriented molecules therein are cut.
In view of eliminating the above drawbacks of the prior art, an object of the present invention is to provide a bone joining device which can be produced with improved productivity in a higher yield.
Another object of the present invention is to provide a method for producing a novel bone joining device having screw threads with greatly improved shear strength, particularly a method for producing a screw-like product.